Catalytic reactor and process

ABSTRACT

Fischer-Tropsch synthesis is performed using a compact catalytic reactor unit ( 10 ) defining channels in which is a gas-permeable catalyst structure ( 16 ), the channels extending between headers ( 18 ). The synthesis occurs in at least two stages, as the reactor unit provides at least two successive channels ( 14, 14   a ) for the Fischer-Tropsch synthesis connected by a header, the gas flow velocity through the first channel being sufficiently high that no more than 65% of the carbon monoxide undergoes conversion. The gases are cooled ( 25 ) in the header between the two stages, so as to condense water vapour, and then pass through the second channel at a sufficiently high gas flow velocity that no more than 65% of the remaining carbon monoxide undergoes conversion. This lowers the partial pressure of water vapour and so suppresses oxidation of the catalyst.

This invention relates to a chemical process, and to catalytic reactorssuitable for use in performing the process.

A process is described in WO 01/51194 (Accentus plc) in which methane isreacted with steam, to generate carbon monoxide and hydrogen in a firstcatalytic reactor; the resulting gas mixture is then used to performFischer-Tropsch synthesis in a second catalytic reactor. The overallresult is to convert methane to hydrocarbons of higher molecular weight,which are usually liquid or solid under ambient conditions. The twostages of the process, steam/methane reforming and Fischer-Tropschsynthesis, require different catalysts, and catalytic reactors aredescribed for each stage. The catalytic reactors enable heat to betransferred to or from the reacting gases, respectively, as thereactions are respectively endothermic and exothermic; the heat requiredfor steam/methane reforming is provided by gas combustion. A knowncatalyst for the Fischer-Tropsch synthesis utilises small particles ofcobalt on a ceramic support, but it has been found that this catalystcan suffer oxidation or an irreversible reaction with the ceramicsupport in the presence of water vapour, with a resultant decrease inactivity. An improved way of performing this process has now been found.

According to the present invention there is provided a process forperforming Fischer-Tropsch synthesis using at least one compactcatalytic reactor unit defining channels for the Fischer-Tropschsynthesis reaction in which there is a gas-permeable catalyst structure,wherein a carbon-monoxide-containing gas undergoes Fisher-Tropschsynthesis in at least two successive stages, the gas flow velocity inthe first stage being sufficiently high that no more than 70% of thecarbon monoxide undergoes the synthesis reaction in the first stage, thegases being cooled between the successive stages so as to condense watervapour, and the gas flow velocity in the second stage being sufficientlyhigh that no more than 70% of the remaining carbon monoxide undergoesthe synthesis reaction in the second stage.

Preferably in both the first stage and the second stage the spacevelocity is above 1000/hr, but preferably no greater than 15000/hr.Preferably the process is operated so that water vapour does not exceed20 mole %. Preferably, in each stage, no more than 65% of the carbonmonoxide undergoes conversion.

The space velocity, in this specification, is defined as the volume flowrate of the gases supplied to the reactor (measured at STP), divided bythe void volume of the reactor. Thus, if the reactor is at 210° C. and apressure of 2.5 MPa, a space velocity of 5000/hr corresponds to a gasflow (at operating conditions) of about 354 times the void volume perhour, and so to a residence time of about 10 s.

Thus the invention also provides a process for performingFischer-Tropsch synthesis on a gas containing hydrogen and carbonmonoxide using at least one compact catalytic reactor unit definingchannels for the Fischer-Tropsch synthesis reaction in which there is agas-permeable catalyst structure, wherein the synthesis reaction isperformed in at least two successive stages, at a sufficiently high gasflow velocity that water vapour does not exceed 20 mole %, and thatbetween successive stages the gases are cooled so as to condense watervapour.

The invention also provides an apparatus for performing such aFischer-Tropsch synthesis. This may be a compact catalytic reactor unitincorporating headers that connect successive flow channels, the headersenclosing means to condense water vapour and to remove condensed liquidsfrom the header. The catalytic reactor unit preferably comprises aplurality of metal sheets arranged as a stack and bonded together todefine channels for the Fischer-Tropsch synthesis alternating withchannels for a heat exchange fluid. Preferably the temperature in thesynthesis channels is above 190° C., for example 200° C. Corrugated ordimpled foils, metal meshes, or corrugated or pleated metal felt sheetsmay be used as the substrate of the catalyst structure within the flowchannels to enhance heat transfer and catalyst surface area.

It will be appreciated that the materials of which the reactor are madeare subjected to a corrosive atmosphere in use. The reactor may be madeof a metal such as an aluminium-bearing ferritic steel, for example itmight comprise iron with 15% chromium, 4% aluminium, and 0.3% yttrium(eg Fecralloy (™)). When this metal is heated in air it forms anadherent oxide coating of alumina which protects the alloy againstfurther oxidation; this oxide layer also protects the alloy againstcorrosion. Where this metal is used as a catalyst substrate, and iscoated with a ceramic layer into which a catalyst material isincorporated, the alumina oxide layer on the metal is believed to bindwith the oxide coating, so ensuring the catalytic material adheres tothe metal substrate. Other stainless steels may also be used. The sheetsdefining the channels may alternatively be of aluminium.

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawings, inwhich:

FIG. 1 shows a sectional view of a reactor suitable for performingFischer-Tropsch synthesis, showing a plate in plan; and

FIG. 2 shows a modification of the reactor of FIG. 1.

The invention relates to Fischer-Tropsch synthesis, which may form partof a process for converting methane to longer chain hydrocarbons.Fischer-Tropsch synthesis is a reaction between carbon monoxide andhydrogen, and this gas mixture may for example be generated bysteam/methane reforming. In Fischer-Tropsch synthesis the gases react togenerate a longer chain hydrocarbon, that is to say:nCO+2nH₂→(CH₂))_(n)+nH₂Owhich is an exothermic reaction, occurring at an elevated temperature,typically between 190 and 350° C., for example 210° C., and an elevatedpressure typically between 2 MPa and 4 MPa, for example 2.5 MPa, in thepresence of a catalyst such as iron, cobalt or fused magnetite, with apromoter. The exact nature of the organic compounds formed by thereaction depends on the temperature, the pressure, and the catalyst, aswell as the ratio of carbon monoxide to hydrogen.

A preferred catalyst comprises a coating of gamma-alumina of specificsurface area 140-450 m²/g with about 10-40% (by weight compared to theweight of alumina) of cobalt, and with a ruthenium/platinum promoter,the promoter being between 0.01% to 10% of the weight of the cobalt.There may also be a basicity promoter such as gadolinium oxide. Theactivity and selectivity of the catalyst depends upon the degree ofdispersion of cobalt metal upon the support, the optimum level of cobaltdispersion being typically in the range 0.1 to 0.2, so that between 10%and 20% of the cobalt metal atoms present are at a surface. The largerthe degree of dispersion, clearly the smaller must be the cobalt metalcrystallite size, and this is typically in the range 5-15 nm. Cobaltparticles of such a size provide a high level of catalytic activity, butmay be oxidised in the presence of water vapour, and this leads to adramatic reduction in their catalytic activity. The extent of thisoxidation depends upon the proportions of hydrogen and water vapouradjacent to the catalyst particles, and also their temperature, highertemperatures and higher proportions of water vapour both increasing theextent of oxidation.

Referring now to FIG. 1 a reactor 10 for Fischer-Tropsch synthesiscomprises a stack of Fecralloy steel plates 12, each plate beinggenerally rectangular, 450 mm long and 150 mm wide and 6 mm thick, thesedimensions being given only by way of example. On the upper surface ofeach such plate 12 are rectangular grooves 14 of depth 5 mm separated bylands 15 (eight such grooves being shown), but there are three differentarrangements of the grooves 14. In the plate 12 shown in the drawing thegrooves 14 extend diagonally at an angle of 45° to the longitudinal axisof the plate 12, from top left to bottom right as shown. In a secondtype of plate 12 the grooves 14 a (as indicated by broken lines) followa mirror image pattern, extending diagonally at 45° from bottom left totop right as shown. In a third type of plate 12 the grooves 14 b (asindicated by chain dotted lines) extend parallel to the longitudinalaxis.

The plates 12 are assembled in a stack, with each of the third type ofplate 12 (with the longitudinal grooves 14 b) being between a plate withdiagonal grooves 14 and a plate with mirror image diagonal grooves 14 a,and after assembling many plates 12 the stack is completed with a blankrectangular plate. The plates 12 are compressed together and subjectedto a heat treatment to bring about diffusion bonding or they are brazedtogether, so they are sealed to each other. Corrugated Fecralloy alloyfoils 16 (only one is shown) 50 microns thick coated with a ceramiccoating impregnated with a catalyst material, of appropriate shapes andwith corrugations 5 mm high, can be slid into each such diagonal groove14 or 14 a.

More preferably pairs of corrugated catalyst-coated foils 16 withcorrugations about 2.4 mm high are stacked together with a flatcatalyst-coated foil between them, and spot welded together, beforebeing slid into the grooves 14 or 14 a.

Header chambers 18 are welded to the stack along each side, each header18 defining three compartments by virtue of two fins 20 that are alsowelded to the stack. The fins 20 are one third of the way along thelength of the stack from each end, and coincide with a land 15 (or aportion of the plates with no groove) in each plate 12 with diagonalgrooves 14 or 14 a. Coolant headers 22 in the form of rectangular capsare welded onto the stack at each end, communicating with thelongitudinal grooves 14 b. In a modification (not shown), in place ofeach three-compartment header 18 there might instead be three adjacentheader chambers, each being a rectangular cap like the headers 22.Within each of the central compartments of the headers 18 there arecoolant tubes 25 that extend the entire height of the stack. At the baseof each of these central compartments is an outlet duct (not shown)through which liquids condensing onto the tubes 25 can emerge. For use,the reactor 10 is arranged with the plates 12 in substantiallyhorizontal planes so that the coolant tubes 25 are substantiallyvertical.

In use of the reactor 10 the mixture of carbon monoxide and hydrogen issupplied to the compartments of both headers 18 at one end (the lefthand end as shown) of the stack, and so gases produced byFischer-Tropsch synthesis emerge through the compartments of bothheaders 18 at the right hand end as shown. The flow path for the mixturesupplied to the top-left header compartment (as shown), for example, isthrough the diagonal grooves 14 into the bottom-middle headercompartment, and then to flow through the diagonal grooves 14 a in otherplates in the stack into the top-right header compartment. A coolant issupplied to the header 22 at the same end of the stack, to maintain thetemperature within the reactor 10 at about 210° C., so that the coolantis at its lowest temperature at the area where heat generation is at itsmaximum during the first stage. Hence the flows of the reacting gasesand the coolant are at least partially co-current. The intention is toapproach isothermal conditions throughout the reactor 10; this has theadvantage of minimising the risk of any wax (i.e. very long chainhydrocarbon) blocking the flow channels towards the outlet from thereaction channels if the local temperature drops below about 190° C..(If wax deposits occur, they may be removed by raising the coolanttemperature by between 5° and 15° C., and feeding hydrogen-rich tail gasthrough the reactor.) The flow rate (space velocity) of the reactinggases is in the range 1000-15000/hr, so as to ensure that the conversionof carbon monoxide is only about 60% or less by the time the gases reachthe middle compartments of the headers 18.

The coolant tubes 25 are supplied with coolant at a differenttemperature so that they are cooler, for example at 150° C. (which isbelow the boiling point of water at the pressure in the reactor).Consequently water vapour (and some of the longer-chain hydrocarbons)condense on the outer surface of the coolant tubes 25, and runs downthose tubes 25 to emerge from the outlet duct (not shown) at the bottomof the stack. This significantly reduces the partial pressure of watervapour in the gas mixture that flows on into the next set of diagonalgrooves 14 or 14 a. The result is that the Fischer-Tropsch synthesistakes place in two successive stages—the first stage being as the gasflows from the inlet compartments of the headers 18 to the middlecompartments; and the second stage being as the gas flows from themiddle compartments to the outlet compartments—and at least part of thewater vapour generated in the first stage is removed from the gas streambefore it enters the second stage.

It will be appreciated that the reactor 10 may be modified in variousways, and that in particular the plates 12 may be of differentthicknesses. For example the plates 12 defining the diagonal grooves 14and 14 a in which Fischer-Tropsch synthesis takes place might be 10 mmthick with grooves 9 mm deep, while the plates 12 with longitudinalgrooves 14 b for the coolant might be only 4 mm thick with 3 mm deepgrooves. In this case the corrugated foils 16 might be replaced by astack of say three or four corrugated foils which may be spot weldedtogether so the overall height is 9 mm. Such deeper grooves provide anadvantage if any waxy material is produced, as they are less vulnerableto blockage. Channels greater than about 2 mm deep improve the bulktransport properties of the corrugated catalyst insert 16; in the caseof Fischer-Tropsch synthesis this enables efficient drainage and removalof liquid products, and transfer of reactant gases to the surface of thecatalyst. The pitch or pattern of the corrugated foils 16 may vary alonga reactor channel 14 or 14 a to adjust catalytic activity, and henceprovide for control over the temperatures or reaction rates at differentpoints in the reactor 10. Furthermore the diagonal grooves may decreasein width, and possibly also depth, along their length, so as to vary thefluid flow conditions, and the heat or mass transfer coefficients.

During the synthesis reaction the gas volume decreases, and byappropriate tapering of the channels 14 the gas velocity may bemaintained as the reaction proceeds, to maintain the target conversion.An alternative way of maintaining the gas velocity is to decrease thenumber of flow channels, as shown in FIG. 2, to which reference is nowmade. This shows a view corresponding to that of FIG. 1. The onlydifference is that the diagonal grooves 14 (and 14 a) defining thesecond stage of the Fischer-Tropsch synthesis, that is to say thegrooves 14 (and 14 a) between the middle compartment and the right handcompartment of the headers 18, are separated by wider lands 30, so thatthere are only three such grooves in each plate 12.

It will also be appreciated that a modified reactor might provide morestages, for example being a three stage Fischer-Tropsch reactor, theheaders 18 defining four successive compartments along each side of thereactor, and with condenser tubes 25 in each of the two middlecompartments. The overall conversion may be substantially the same, forexample two 60% conversion stages and three 50% conversion stages wouldeach provide an overall conversion above 80%.

Removal of the water vapour and the lower boiling point hydrocarbonsonto the condenser tubes 25 not only lowers the partial pressure ofwater vapour and so suppresses the oxidation of the catalyst, but hasthe further benefit of removing at least some of those hydrocarbons thatwould form a liquid layer on the catalyst structure. Any such a liquidlayer inhibits contact of the gas mixture with the catalyst particlesand inhibits diffusion of the product hydrocarbons away from thecatalyst particles, so removal of the hydrocarbons liquid minimisesthese diffusional resistances.

In FIGS. 1 and 2 only four condenser tubes 25 are shown in each middlecompartment, but it will be appreciated that there might be a differentnumber of tubes, for example ten or more. And to enhance heat transfereach tube 25 may be provided with fins, preferably fins extendinglongitudinally so that flow of condensed liquid down the tube 25 is notimpeded. Not only does water vapour condense on the tubes, but anyliquid droplets entrained with the gas flow tend to impact with thesurface of the tubes 25 and so are disengaged from the gas flow. As analternative to the heat exchanger tubes 25 or other heat transfersurfaces, a spray condenser system may be provided within the middlecompartments of the headers 18, which might use as the coolant fluidrecycled products from the Fischer-Tropsch synthesis at about 150° C.This would be particularly beneficial if there is a risk of wax depositsfouling the heat exchanger surfaces.

Alternatively the cooling and condensation may be carried out separatefrom and outside the reactor. For example three reactors 10 as shown inFIG. 1 but without the cooling tubes 25 in the header might be arrangedto carry gas flows in parallel, the conversion of CO being restricted tobelow 65% by controlling the reaction temperature and space velocity.The outlet gases from the three reactors are connected via a manifold toa condenser unit in which the water vapour and liquid hydrocarbonproduct is condensed. The remaining gases, with lowered water partialpressure, might then be supplied to a single such reactor 10 (againwithout the cooling tubes 25), so that again about 60% of the residualunreacted CO undergoes the synthesis reaction. The decrease in gasvolume between the first stage and the second stage—because much of thegas has undergone synthesis and formed a liquid—is accommodated byreducing the number of reactor units from three to one, so as tomaintain a high flow velocity.

Additional benefits of the high gas flow velocity are a reduction in thetemperature variation across the reaction channels, by helping toredistribute the heat from the exothermic reactions at the surface ofthe catalyst into the gas phase. It also helps to entrain the liquidreaction products into the gas flow and to keep the catalyst surfacefree of waxy deposits.

1. A process for performing Fischer-Tropsch synthesis using at least onecompact catalytic reactor unit defining channels for the Fischer-Tropschsynthesis reaction in which there is a gas-permeable catalyst structure,characterized in that a carbon monoxide-containing gas undergoesFischer-Tropsch synthesis in at least two successive stages, the gasflow velocity in the first stage being sufficiently high that no morethan 70% of the carbon monoxide undergoes the synthesis reaction in thefirst stage, the gases being cooled between the successive stages so asto condense water vapour, and the gas flow velocity in the second stagebeing sufficiently high that no more than 70% of the remaining carbonmonoxide undergoes the synthesis reaction in the second stage.
 2. Aprocess as claimed in claim 1 performed using a single reactor unit,wherein each stage of the synthesis reaction takes place in a set ofchannels within the reactor unit, and the gases are cooled within aheader between successive stages.
 3. A process as claimed in claim 1wherein a carbon monoxide-containing gas stream flows through aplurality of first channels in parallel in the first stage, and thenthrough a plurality of second channels in parallel in the second stage,the cross-sectional area of the plurality of second channels being lessthan that of the plurality of first channels.
 4. A process as claimed inclaim 3 wherein the number of second channels is less than the number offirst channels.
 5. A process as claimed in claim 3 wherein in both thefirst stage and the second stage the space velocity is above 1000/hr. 6.A process as claimed in claim 5 wherein, in both the first stage and thesecond stage the space velocity is no greater than 15000/hr.
 7. Aprocess as claimed in claim 5 wherein water vapour does not exceed 20mole %.
 8. A process as claimed in claim 5 wherein the gas flow velocitythrough both the first stage and the second stage is sufficiently highthat no more than 65% of the carbon monoxide undergoes the synthesisreaction.
 9. A process for performing Fischer-Tropsch synthesis on a gascontaining hydrogen and carbon monoxide using at least one compactcatalytic reactor unit defining channels for the Fischer-Tropschsynthesis in which there is a gas-permeable catalyst structure, whereinthe synthesis is performed in at least two successive stages, at asufficiently high gas flow velocity that water vapour does not exceed 20mole %, and that between successive stages the gases are cooled so as tocondense water vapour.
 10. Apparatus for performing a Fischer-Tropschsynthesis as claimed in claim 1, comprising at least one compactcatalytic reactor unit defining channels for the Fischer-Tropschsynthesis reaction in which there is a gas-permeable catalyst structure,connecting means communicating between successive sets of channels, andcooling means within the connecting means to condense water vapour andto remove condensed liquids from the gas flow.
 11. Apparatus as claimedin claim 10 wherein the successive sets of channels are in the samereactor unit, and the connecting means is a header.
 12. Apparatus asclaimed in claim 10 wherein the cross-sectional area of the flowchannels carrying flow out of the connecting means is less than thecross-sectional area of the flow channels carrying flow into theconnecting means.
 13. Apparatus as claimed in claim 10 wherein thenumber of flow channels carrying flow out of the connecting means isless than the number of flow channels carrying flow into the connectingmeans.
 14. Apparatus as claimed in claim 10 also comprising means toensure the temperature in the synthesis channels does not exceed 210° C.